The present invention relates to an electroluminescent device (hereinafter referred to as xe2x80x9cEL devicexe2x80x9d) and a method for producing the same. In particular, the present invention relates to an EL device which uses a prismatic film effectively, and a method for producing the same.
EL devices are known, which comprise a semiconductor luminescent layer or a so-called xe2x80x9cdispersion type luminescent layerxe2x80x9d that is formed by dispersing luminescent particles such as fluorescent substances in a matrix resin such as a polymer having a high dielectric constant.
For example, an EL device comprising a xe2x80x9cdispersion type luminescent layerxe2x80x9d can be formed by laminating a transparent substrate, a transparent electrode layer, an insulating layer consisting of a vinylidene fluoride base matrix resln, a luminescent layer comprising a vinylidene fluoride base matrix resin and fluorescent particles, the same insulating layer as above, and a rear electrode in this order, as disclosed in Japanese Patent Publication JP-B-59-14878.
Japanese Patent Publication JP-B-62-59879 discloses an EL device comprising a polyester film, an ITO electrode, a luminescent layer comprising a cyanoethylated ethylene-vinyl alcohol copolymer (a matrix resin) and fluorescent particles, and an aluminum toil (a rear electrode), which are laminated in this order.
In general, the both surfaces (a surface and back surface) of the transparent substrate which emits light from the luminescent layer are flat and smooth in these EL devices, and no means is provided for increasing the luminance through an optical function.
It is known from Japanese Patent Publication JP-A-5-94994 and the like that the luminance can be increased by placing a prismatic film having a surface carrying a plurality of prisms on the light-emitting surface of a planar luminescent body such as an EL device and utilizing the optical functions of the prisms.
The above structure can intensify the light emitted in the normal direction from the light-emitting surface, but cannot intensity the light effectively in the direction deviating from the normal line (for example, deviating by at least 30 degrees from the normal line). Thus, the increase of the luminance in a wide observation angle range cannot be expected. The reason for this has been found according to a results of the research by the present inventors. That is, the light in the direction deviating from the normal line is easily reflected by an air layer present at the interface between the luminescent layer and prismatic film, and an amount of light which reaches the prismatic film decreases.
The present invention provides an EL device which intensifies the light emitted in the normal line direction and also in the directions deviating from the normal line, and increases the luminance in the wide observation angle range.
Accordingly, the present invention provides an electroluminescent device comprising an electroluminescent element (9) having a light-emitting surface (91), and a prismatic film (1) having a surface (11) on which a plurality of prisms are provided and a back surface (12) opposed to said surface (11) and being placed on said light-emitting surface (91), wherein the back surface (12) of the prismatic film (1) and said light-emitting surface (91) of the electroluminescent element (9) are closely bonded optically.
The EL device of the present invention can effectively intensify the light emitted in the normal line direction from the light-emitting surface and directions deviating from the normal line, and increases the luminance in the wide observation angle range, since the back surface of the prismatic film (1) (that is, the surface having no prisms) and the light-emitting surface of the EL device (9) are closely bonded optically.
The prismatic film (1) and EL device (9) being xe2x80x9cclosely bonded opticallyxe2x80x9d means that no air layer, which functions to decrease the amount of light which emitted from the EL device and reaches the prismatic film, is present between them. Such the closely bonded state can be easily formed particularly when the light-emitting surface of the EL device (9) is substantially flat, and also the back surface of the prismatic film (1) is substantially flat.
The optically closely bonded state can be achieved by:
A: coating the transparent conductive layer (2) directly on the back surface of the prismatic film (1) and then forming the luminescent layer (8) in close contact with the back surface of the transparent conductive layer (2), or
B: adhering the prismatic film (1) and the EL element (9) through a transparent adhesive layer with leaving substantially no air layer between them.
In the latter case, the light transmission of the transparent adhesive should be at least 75%, preferably at least 80%, in particular at least 85%. The transparent adhesive may be an acrylic pressure-sensitive adhesive, acrylic heat-sensitive adhesive, etc.
Herein, the xe2x80x9clight transmissionxe2x80x9d means a transmission of light measured using a UV/visible light spectrometer xe2x80x9cV-560xe2x80x9d manufactured by Nippon Bunko Kabushikikaisha at a wavelength of 550 nm.
The filling rate of the luminescent particles is easily increased and thus the luminance is further increased, when the luminescent layer (8) includes the luminescent particle layer (4) which consists essentially of particles containing the luminescent particles (7), is placed between the support layer (3) and the insulating layer (5) and is in close contact with the support layer (3) and the insulating layer (5).
The EL device can be advantageously produced by a method comprising the steps of:
i) providing a prismatic film (1) on the back surface (11) of which a transparent conductive layer (2) is laminated,
ii) applying a paint for forming a support layer (3) containing matrix resin on said transparent conductive layer (2),
scattering particles containing luminescent particles (7) in a layer state, and embedding a part of each particle in said paint prior to solidification of said paint,
then, solidifying said paint and forming a transparent support layer (3) and a luminescent particle layer (4) bonded to said support layer (3),
iii) applying a paint for forming an insulating layer (5) comprising an insulating material on said luminescent particle layer (4), solidifying said paint and forming said insulating layer (5) bonded to said luminescent particle layer (4), and
iv) laminating a rear electrode (6) on said insulating layer (5).
This method can produce a sheet-form EL device having a high luminance and a large area, without using a dispersion paint of luminescent particles.